Alumina coatings on aluminum alloy were prepared by micro-arc oxidation (MAO) process using DC and AC power supplies, respectively. In comparison with the coating deposited by DCMAO, the influence of the cathodic current on the composition, structure and properties of the ACMAO coating was investigated. It is found that the coating deposited by DCMAO is composed of alpha-Al2O3, whereas the coating deposited by ACMAO has a mixture composition of alpha-Al2O3 and gamma-Al2O3. The results of properties show that compared with the coatings deposited by DCMAO, the ACMAO coatings possess higher density, hardness and corrosion resistance. It can be attributed to that the DCMAO coating is rougher and existing much more micro-cracks in its inner layer. As a result, the adhesion of the DCMAO coating to the substrate decreases. (c) 2005 Elsevier B.V. All rights reserved.